In addition to low molecular weight hydrophobic drugs which have been mainly employed so far, hydrophilic bioactive substances such as peptides and nucleic acids have recently appeared in clinical sites. They are employed as pharmaceuticals and exhibit remarkable therapeutic effects. However, up to now, in most cases, their administration methods are limited to usage as injection solutions. This is because hydrophilic substances are incapable of passing through the epithelial cell layer of mucosa, unlike known drugs. Since the cells in the epithelial cell layer are closely bound to each other to prevent invasion of foreign substances into the living body, substances having hydrophilicity can hardly pass through the layer, so that oral administration, which is a generally-used administration method of drugs, is incapable of allowing the substances to act inside the body.
Since administration of a drug by injection is a heavy burden on a patient and a physician, particularly in cases where the therapy is frequent and continues for a long time, various methods have been studied to enable administration of such a hydrophilic bioactive substance by a method other than injection.
In oral administration which is widely used as a general method for administration of a drug, there are a number of attempts for technologies to allow absorption of a hydrophilic bioactive substance. In particular, to improve permeability of the intestinal epithelial cell layer which is a major region of absorption when oral administration is carried out, attempts have been made using surfactants, epithelial cell-adhesive substances, cell penetrating peptides and the like.
“Cell penetrating peptide” is a general term for peptides having a property to migrate from outside a cell to inside the cell without destroying the cell membrane. Well-known examples of the peptides include various types of peptides such as oligoarginine having a stretch of arginine; Tat, a peptide of the HIV-1 virus (Patent Literature 1); and penetratin having the same amino acid sequence as shown in SEQ ID NO:1 of the present invention (Patent Literatures 2 and 3). Various peptides are included therein, and examples thereof include those characterized by simple basicity; those having a hydrophobic domain; those characterized by amphiphilicity of the primary structure or secondary structure of the peptides; and those having an uncertain mechanism. Using these peptides, researches on their migration capabilities into the cell, and their uses, as vehicles, for delivery of genes linked to them into the cell have been extensively carried out.
Further, studies aiming to promote penetration through the epithelia cell layer and to promote absorption upon oral administration using such a property have been carried out (Patent Literatures 4 and 5). However, since the absorption promotion effects of the cell penetrating peptides used in these studies are based on evaluation with rinsed intestinal tract and the like, the actual effects upon oral administration are not clear. Further, it is difficult to realize stable absorption because of digestive degradation in stomach and changes in the environment in the gastrointestinal tract by ingestion, so that practical application of oral administration of a hydrophilic bioactive substance has not been achieved.
As a method other than oral administration to realize the absorption, an administration method employing the nasal mucosal layer has been devised. Between nasal mucosa and mucosa of the gastrointestinal tract, there are differences in the types of the cells constituting the mucosal layer and differences in properties of individual cells, and degradative enzymes existing in the lumens show various differences such as differences in their types and amounts, so that technologies other than those for promotion of absorption in the gastrointestinal tract are demanded for promotion of transnasal absorption.
Administration can be easily carried out through nasal mucosa, and nasal mucosa has properties suitable for absorption of drugs because of developed blood vessels and lymph vessels under it. Further, it is considered that, since the drug absorbed from the nasal cavity directly passes into the general circulation after the absorption, the first-pass effect by liver can be avoided, so that the nasal cavity is considered to be effective as an administration site for drugs prone to be metabolized in the gastrointestinal tract and/or liver. However, since, as in the other absorption sites, nasal mucosa has low permeabilities for hydrophilic bioactive substances, and since the area of the absorption site is small, an absorption-promoting technology is required to realize absorption of hydrophilic bioactive substances from the nasal cavity.
There are examples of clinical tests with transnasal formulations using surfactants as absorption-promoting agents by a plurality of drug makers so far, but all of the tests were given up because of strong irritation to nasal mucosa. Further, as a method which does not employ a surfactant, promotion of absorption using a peptide has been attempted at the research phase, and an attempt to promote transnasal absorption by direct linking of a peptide to a drug is disclosed in Patent Literature 6. However, since this technology requires chemical modification of a drug, it necessarily has many problems to be solved such as decrease in the pharmacological activity of the drug, changes in the pharmacokinetics, increase in the production cost and antigenicity of the administered drug.
Possibilities of promotion of transnasal absorption by penetratin or a modified product thereof are mentioned in Patent Literature 2 and 3, but since these require covalent bond between the drug to be allowed to permeate the mucosa and the modified product of penetratin, they are technologies different from the present invention wherein a hydrophilic bioactive substance and penetratin or a modified product thereof are not covalently bonded to each other. It is known that efficient occurrence of desorption from the cell, in addition to migration from outside the cell to inside the cell, is required to realize promotion of nasal absorption using a cell penetrating peptide, and the absorbability upon absorption through nasal mucosa is affected by various factors other than cell permeability such as degradability of the peptide by various degradative enzymes existing in the nasal mucosal tissue. Therefore, necessity for confirmation, by experiments using a model animal or the like, of whether or not the cell penetrating peptide has permeability through nasal mucosa is common general technical knowledge of those skilled in the art. However, since Patent Literatures 2 and 3 do not disclose experimental verification on nasal mucosal permeability of penetratin and the modified product thereof, it is not easy for those skilled in the art to infer from Patent Literature 2 and 3 that penetratin and the modified product thereof have nasal mucosal permeability.
There is also an attempt to administer oligoarginine which is not covalently linked to the drug as a nasal absorption-promoting substance together with the drug (Patent Literature 7), but this requires a high concentration of oligoarginine and its effect has been demonstrated with only a single model drug, fluorescently-labeled dextran, so that permeability of a peptide hydrophilic bioactive substance is unknown.
Thus, a highly-practical technology to enable high efficiency nasal absorption of hydrophilic bioactive substances has not been discovered yet.
[Patent Literature 1] JP 10-33186 A
[Patent Literature 2] Japanese Translated PCT Patent Application Laid-open No. 2002-530059
[Patent Literature 3] Japanese Translated PCT Patent Application Laid-open No. 2002-519392
[Patent Literature 4] JP 2006-257074 A
[Patent Literature 5] JP 2008-7448 A
[Patent Literature 6] WO 2004/037859
[Patent Literature 7] JP 10-95738 A